In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Since the early 1980's, golden-coloured TiN-layers have been widely used for surface protective applications. In order to improve the oxidation resistance of these coatings, work began in the mid-1980's with adding aluminium to TiN [see, e.g.—H. A. Jehn, et al, J. Vac. Sci. Technol. A 4, 2701 (1986) and O. Knotek et.al, J. Vac. Sci. Technol. A 4, 2695 (1986)]. The compound thus formed, cubic-phase Ti1−xAlxN, was found to have superior oxidation resistance and enabled greater cutting speeds during machining, prolonged tool life, machining of harder materials, and improved manufacturing economy.
The mechanisms responsible for the excellent cutting performance of Ti1−xAlxN-coated tools have this far been associated with the coating's oxidation resistance. B.-J. Kim et.al., J. Vac. Sci. Technol. A 17(1), 133 (1999) reported that an increased aluminium content in the Ti1−xAlxN compound improves the oxidation resistance. TiN oxidises rapidly at temperatures of 500–600° C. according to W. D. Münz, invited paper Int. Conf. Met. Coat., San Diego, USA (1986) and H. G. Tompkins, J. Appl. Phys. 70, 3876 (1991), whereas oxidation of Ti1−xAlxN starts at 750–900° C. [D. McIntyre et. al., J. Appl. Phys. 67, 1542 (1990)]. This gives an increased upper operating temperature of Ti1−xAlxN compared to TiN from 450–500° C. to 750-800° C. according to Münz et al This concept of mainly Ti1−xAlxN based materials, has been a subject for a large number of further optimization of different types, like macroscopically compositional gradients over the coated components as U.S. Pat. No. 5,272,014 discloses. Another way of optimization has been by applying different concepts of multilayer as; alternating Ti and Al containing layers (U.S. Pat. No. 6,309,738), oxygen and non-oxygen containing layers (U.S. Pat. No. 6,254,984), one of the layers stacked in the multilayer consists itself of a multilayer (U.S. Pat. No. 6,077,596), alternating nitrogen content (U.S. Pat. No. 5,330,853) or using one metastable compound (U.S. Pat. No. 5,503,912) or as an aperiodic multilayer (U.S. Pat. No. 6,103,357).
H. Holleck, Surf. Coat. Technol. 36, 151 (1988) has reported that the solid solubility of AlN in TiN is extremely low, and only at 2,700K it reaches ˜5 mol %. For larger AlN fractions, or at lower temperatures, the equilibrium system consists of cubic TiN and hexagonal AlN. However, as is well known, Ti1−xAlxN can be deposited as a metastable cubic structure by using physical vapour deposition (PVD). At an elevated temperature during heat treatment or operation of a coated cutting tool, enough energy may then be supplied that phase separation into c-TiN and h-AlN occurs which normally deteriorates the wear resistance of the coating.